External modulators are commonly used in analog fiber optic communication systems, such as CATV signal distribution, to modulate in amplitude an optical carrier with an information signal. The advantages of using external modulation in analog fiber optic systems include excellent performance stability, flexible system adaptability, and minimization of residual FM chirp while reducing fiber-induced dispersion effects.
Adaptive control of transmission systems employing single stage MZI external modulators has received much attention in the last years. FIG. 1 illustrates a typical control system including a high power continuous-wave (CW) laser connected to an external optical modulator, which is a Mach-Zehnder interferometer (MZI) whose output is connected to an optical coupler so that a sample of the optical output signal can be used for closed loop distortion cancellation.
Typical MZI modulators include an electrical input port for application of the desired information carrying signals that are to be transmitted along with a DC bias port used to apply an appropriate DC bias voltage in order to maintain the modulator's operating point at quadrature. The transfer function of a Mach-Zehnder modulator is typically a raised cosine function, a portion of which is shown in FIG. 2, where the quadrature point is indicated.
Maintaining the modulator at quadrature is very important for the reduction of even order distortion of the information carrier. Furthermore, MZI optical modulators fabricated in lithium niobate (LiNbO3) have been shown to be sensitive to thermal and mechanical stresses that cause dynamic shifts of the quadrature bias point. The variations induced by stresses require active control to maintain an optimum distortion performance. Assuming that a proper bias control is possible, odd order distortion generation becomes dominant in systems employing these electro-optic devices.
Odd order distortion performance improvement has been traditionally achieved using electrical pre-distortion techniques, as proposed in U.S. Pat. No. 5,161,044. The described technique employs non-linear circuits elements to create odd order distortion products of the information signal, which are equal in amplitude and opposite in phase with the products generated by non linear transfer function of a Mach-Zehnder modulator. It has been shown that the amount of odd order cancellation obtainable using this technique is dependent on the accuracy of the pre-distortion circuitry's ability to match the modulator's RF response over the information signals bandwidth.
FIG. 3 shows a typical closed loop control system suitable to adaptively maintain even and odd order distortion reduction. Three pilot tones at frequencies f1, f2 and f3 are applied to an MZI modulator trough the informational carrying input port or through the DC bias input port, so that the optical signal output from the modulator contains the applied pilot tones and their resultant non-linear products. An optical coupler samples the optical output signal, by splitting a portion of the signal, which is then detected by an optical to electrical converter, such as a photodetector. The signal sample is analyzed for even and odd distortion products induced upon the pilot tone frequencies and the produced error signals are used to control the DC bias point of the modulator in order to reduce even order distortions and the amplitude and phase of the electrical predistorter in order to reduce odd order distortions.
In an attempt to improve the accuracy of the correction of the system non linearity, the inventor has considered to increase the amplitude of the pilot tones in order to increase the level of the produced error signal. However, the inventor has understood that increasing the amplitude of the pilot tones has undesirable effects on the information carrying signal. The added pilot tone signals will non-linearly combine with the information carrying signal, thereby generating additional undesirable spurious distortion products that degrade the spectral fidelity of the transmitted information carrying signal.
Another possible detrimental effect of adding large pilot tones to the information carrying signal is the reduction in the amount of optical modulation index (OMI) available. The reduction of available OMI combined with the inherent electrical system and optic transmission noise contributions to the signal can reduce the signal to noise ratios (SNR) of the system.
U.S. Pat. No. 6,392,779 discloses a bias control system for automatically controlling a bias point of an electro-optical modulator, the system including a pilot tone generator that generates a first pilot tone at a first frequency and a second pilot tone at a second frequency. The first and second tones are swept in frequency over a predetermined frequency range with a predetermined sweep rate, to spectrally spread third order distortion products over a larger frequency band, allowing the amplitude of the pilot tones to be increased and thereby increasing the gain of the bias control circuit.
An alternative electro-optic modulator topology aimed to improve the linearity performance are the so-called optically linearized modulators (OLMs), which typically comprise two Mach-Zehnder interferometers connected in either a series or a parallel configuration by using an optical coupler. A simplified diagram of a single-stage MZI modulator and a simplified diagram of a series-connected optically linearized modulator (OLM) are shown in FIGS. 4a and 4b, respectively. The basic operation of an OLM can be simply described as a primary MZI modulator optically coupled to a secondary MZI modulator by means of an optical coupler, wherein the secondary MZI is especially designed to substantially cancel the odd order distortion created by the primary MZI. The use of an OLM has the advantage that odd order distortion cancellation occurs in the optical domain and thus its performance is inherently broadband.
However, the inventor has noted that the OLM requires that the two MZI sections within the OLM are simultaneously driven with the same information carrying signal at a given amplitude, phase, and electrical delay for realization of the optical linearization. In addition, each modulator section should be biased independently and at quadrature, to minimize odd order distortions. Therefore, the use of an OLM for external modulation generally require a significantly more complex closed loop control system to achieve an improved performance.
A non-ideal DC bias point setting in the primary MZI could be in principle easily compensated by a corresponding non-ideal DC bias point in the secondary MZI section. The inventor has noted that, although there may be an infinite number of DC bias voltage input combinations that result in a minimized even order distortion solution at the output of the OLM, there exists basically only one solution that results in a minimized odd order distortion output occurs when each individual modulator is correctly biased at its quadrature operating point. Furthermore, the DC bias points of the modulators may vary due to optical wavelength sensitivities, temperature variations, internal piezoelectric electrostatic charge accumulation and long term aging effects of the optical waveguides.